FIG. 1 is a plan transparent view illustrating a main part of a wiring substrate 100 of a related art example. FIG. 2 is a cross-sectional view illustrating a main part of the wiring substrate 100 of a related art example. It is to be noted that some of the components/elements illustrated in FIG. 2 are omitted in FIG. 1. The cross-sectional view of FIG. 2 is taken along line A-A of FIG. 1.
With reference to FIGS. 1 and 2, the wiring substrate 100 has a reference layer 170, an insulation layer, and wirings 150, 160 layered thereon.
The insulation layer 110 includes a glass cloth 120 and a thermosetting resin 130. The glass cloth 120 includes glass fiber bundles 120a, 120b. The glass fiber bundles 120a, 120b are impregnated in the thermosetting resin 130. The glass cloth 120 is formed by plain-weaving the glass fiber bundles 120a, 120b in a grid pattern in which the glass fiber bundles 120a are arranged in parallel with an X axis and spaced apart from each other at substantially constant intervals and the glass fiber bundles 120b are arranged in parallel with a Y axis and spaced apart from each other at substantially constant intervals. The holes (i.e. so-called basket holes) 120x created by forming the glass fiber bundles 120a, 120 are filled with the thermosetting resin 130. No glass fiber exists inside the holes 120x. 
The wirings 150, 169 are selectively formed on a part(s) of a surface on one side of the insulation layer 110, and the reference layer 170 are formed substantially on the entire surface on the other side of the insulation layer 110. The wirings 150, 160 are conductors having predetermined electric signals flowing therethrough. The reference surface 170 is a conductor serving as a return circuit of the predetermined electric signals that flow through the wirings 150, 160.
The wirings 150, 160, which are arranged alongside each other, are differential wirings used for differential signaling. In general, differential signaling can achieve satisfactory high speed transmission. The wiring 150 is positioned on top of the glass fiber bundle 120b. The wiring 160 is positioned in-between adjacent glass fiber bundles 120b. 
As described above, a wiring substrate of a related art example has glass fiber bundles woven as a grid pattern at a stage where the wiring substrate is a material. Because the wiring substrate of the related art example, in general, has differential wirings arranged in X and Y directions, the differential wirings are arranged in parallel or orthogonal with respect to the direction in which the glass fiber bundles are arranged. This is noticeable particularly for differential wirings used for connecting electronic components (e.g., LSI (Large Scale Integration).
Because wirings are becoming narrower compared to the pitch of glass fiber bundles, there is a case where one group of the differential wirings are arranged above the glass fiber bundles while another group of the differential wirings are arranged in-between the glass fiber bundles (i.e. above the thermosetting resin) as illustrated in FIGS. 1 and 2. Because dielectric constant of the differential wirings above the glass fiber bundles and dielectric constant of the differential wirings in-between the glass fiber bundles are different due to difference of glass fiber density (areas where glass fiber is dense and area where glass fiber is sparse), the signal transmission rate of the differential wirings above the glass fiber bundles is slower than that of the differential wirings in-between the glass fiber bundles. Therefore, in a case where one group of differential wirings is arranged above the glass fiber bundles while the other group of differential wirings is arranged in-between the glass fiber bundles (i.e. above the thermosetting resin), problems such as inconsistency of characteristic impedance and difference of transmission delay time occur.
In other words, in a case of transmitting voltage where one group of differential wirings are arranged above the glass fiber bundles while the other group of differential wirings are arranged in-between the glass fiber bundles (i.e. above the thermosetting resin), differential voltage cannot be initiated due to time lag between the differential wirings at a signal receiving side. This time lag tends to be noticeable particularly when transmission distance is long and is a problem for high speed transmission. As a countermeasure to this problem, there is, for example, a method of reducing the influence of inconsistent glass fiber density (dense/sparse areas of glass fiber) by arranging the differential wirings diagonally with respect to the glass fiber bundles arranged as a grid pattern.
However, terminals of a typical electronic component are arranged as a rectangle. Thus, it is difficult to arrange differential wirings diagonally with respect to glass fiber bundles arranged as a grid pattern. Further, although it is possible to weave glass fibers of a wiring substrate material diagonally, the glass fibers of the wiring substrate material are to be cut out diagonally in a case where the wiring substrate material is a ready-made belt-like material. This results to waste of the wiring substrate material. Further, in the case of weaving the glass fibers diagonally, there is a problem of requiring new investment in plant and machinery. In any event, substrate manufacturing conditions is to be reconsidered in a case of fabricating glass fibers diagonally with respect to the outer configuration of the substrate because factors such as shrinkage rate of resin during a substrate laminating process or a perforating process are different compared to a substrate manufacturing method of a related art example.